April 7, 2016
Mallika Marshall, MD, Contributing Editor
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Breast cancer is the most commonly diagnosed cancer among women in the United States, and thyroid disorders affect millions of American women. Most breast cancers are sensitive to hormones such as estrogen, and researchers believe that thyroid hormones have estrogen-like effects. So for years, scientists have wondered whether too much thyroid hormone might play a role in breast cancer. A new study suggests the answer may be “yes.”
It’s all about the thyroid gland
The thyroid gland is a butterfly-shaped gland located at the base of the neck that produces thyroid hormones. Thyroid hormones affect almost every cell in the body and have many important functions, including controlling metabolism, heart rate, and body temperature.
Some people have hyperthyroidism, or an overactive thyroid, in which the thyroid produces too much thyroid hormone. It can cause weight loss, thinning hair, sweating, anxiety, and rapid heart rate. Women have 5-10 times more thyroid activity than men.
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Hypothyroidism, or an underactive thyroid develops when the thyroid gland does not produce enough thyroid hormone. Symptoms of an underactive thyroid include weight gain, fatigue, constipation, depression, dry hair, and a slow heart rate. Like hyperthyroidism, it’s also more common in women than in men.
In an effort to determine whether having an overactive or underactive thyroid affects a woman’s risk of breast cancer, researchers looked at a large group of women in Denmark diagnosed with thyroid disease between 1978 and 2013. More than 60,000 of the study participants had an underactive thyroid, and more than 80,000 had an overactive thyroid. They followed the patients for five to seven years and found that those with an overactive thyroid had a slightly increased risk of breast cancer — an 11% higher risk, to be specific. However, women with an underactive thyroid had a 6% drop in their breast cancer risk.
Putting the study results into perspective
While this study might lead some to conclude that having excess thyroid hormone promotes the development of breast cancer, experts caution that this study does not prove cause and effect. While there may be an association, it is not clear that an overactive thyroid actually causes breast cancer to develop.
Some critics wonder whether women at risk for hyperthyroidism may also be at risk for breast cancer, or whether the treatment for hyperthyroidism may be to blame for increased cancer risk. Another theory is that women with an overactive thyroid see their doctors more often and therefore are more likely to get screened for other problems like breast cancer.
The effect of thyroid hormone on breast cancer risk clearly requires further study. In the meantime, women with an overactive thyroid should stay in close communication with their doctors and follow routine breast cancer screening recommendations.
